Referring to FIG. 1, in a scanned-image system 10 that sweeps a beam 12 to generate or capture an image (not shown in FIG. 1), the image may exhibit noticeable distortion if the pixel clock (not shown in FIG. 1) is not synchronized to the position of the beam.
Assume, for example, that the scanned-image system 10 is a bi-directional image-generating system. A beam sweeper, such as a microelectromechanical (MEMS) mirror 14, rotates back and forth about an axis 16 to sweep the beam 12 through a scan angle 2θ such that each left-to-right and right-to-left sweep of the beam generates a respective line of the image (not shown in FIG. 1). In an image plane 18, which may be occupied by a display screen or, in the case of a virtual retinal display, by a viewer's retina, the mirror 14 sweeps the beam 12 through a scan distance D. Although in this example the generated image is described as spanning the entire scan distance D, the image may span only a portion of the scan distance as discussed below in conjunction with FIGS. 2-4. Furthermore, although three instances of the swept beam 12 are shown, it is understood that the beam may be in only one position at any one time. According to alternative embodiments, a plurality of beams 12 may be swept across the image plane 18.
The pixel clock (not shown in FIG. 1) dictates the pixel that the beam 12 generates at a particular time; therefore, if the pixel clock is synchronized with the position of the beam, then the pixel clock causes the beam to generate a left-most pixel of the image (not shown in FIG. 1) when the beam is in the left-most position L, a center pixel when the beam is in the center position C, and a right-most pixel when the beam is in the right-most position R.
But if the pixel clock is not synchronized with, i.e., is out of phase with, the position of the beam 12, then the generated image may be distorted to a degree that is proportional to the phase error between the pixel clock and the beam. As an extreme example of this phase-error distortion, assume that the phase of the pixel clock lags the position of the beam 12 by D/2 during a left-to-right sweep of the beam; consequently, the pixel clock causes the beam to generate a left-most pixel of the image when the beam is in the center position C, and to generate a center pixel when the beam is in the right position R. And during the following right-to-left sweep of the beam 12, the pixel clock causes the beam to generate the right-most pixel of the previous line when the beam is in the center position C of the current line.
Referring to FIGS. 2-4, an example of the image distortion resulting from a phase error in a pixel clock (not shown in FIGS. 2-4) of the scanned-image system 10 (FIG. 1) is discussed in more detail. In this example, the MEMS mirror 14 resonates back and forth from left to right, thus causing the position of the beam 12 to be sinusoidal relative to time as shown in FIG. 2.
FIG. 2 is a plot of the horizontal position of the beam 12 in the image plane 18 (FIG. 1) versus time over one sweep period T (one left-to-right sweep followed by one right-to-left sweep), with horizontal position being plotted on the vertical axis. Image fields 20 and 22, which the beam respectively generates during the left-to-right and right-to-left sweeps, indicate respective field positions for a pixel-clock phase error of zero. Image fields 24 and 26, which the beam 12 respectively generates during the left-to-right and right-to-left sweeps, indicate respective field positions for an illustrative non-zero pixel-clock phase error.
FIG. 3 is a plan view of an undistorted image frame 28 formed in the image plane 18 by the interleave of the left-to-right and right-to-left image fields 20 and 22 of FIG. 2 for a zero pixel-clock phase error.
FIG. 4 is a plan view of a distorted image frame 30 formed in the image plane 18 by the interleave of the left-to-right and right-to-left image fields 24 (solid line) and 26 (dashed line) of FIG. 2 for a nonzero pixel-clock phase error.
Referring to FIGS. 1-3, assume that one desires to generate within a scan region 31 of the image plane 18 an image frame 28, which includes a vertical line 32 located at a horizontal position between D/5 and 4D/5; therefore, the frame 28 has a width W=3D/5 and is horizontally centered within the scan region. The scan region 31 is defined in the horizontal dimension by the scan distance D and in the vertical dimension by a scan distance V. Borders 34a and 34b between the sides of the scan region 31 and the image frame 28, here indicated as being D/5 wide, may be included, for instance to reduce raster-pinch distortion or provide one or more other advantages.
Because the pixel-clock phase error equals zero, during each left-to-right sweep of the beam 12 the pixel clock may cause the beam to generate a respective horizontal line of the left-to-right image field 20 for a duration Timagefield between times t1 and t2, which respectively correspond to the beam positions D/5 and 4D/5. Some of the horizontal sweeps of the left-to-right image field 20 include segments 36 of the vertical line 32.
Likewise, during each right-to-left sweep of the beam 12, the pixel clock may cause the beam to generate a respective horizontal line of a right-to-left image field 22 for the duration Timagefield between times t3 and t4, which also respectively correspond to the beam positions 4D/5 and D/5. Some of the horizontal sweeps of the right-to-left image field 22 include segments 38 of the vertical line 32.
Referring to FIG. 3, because both of the image fields 20 and 22 are centered within the scan region 31 and have the same horizontal width W and vertical height H, these fields are aligned in the both the horizontal and vertical dimensions; consequently, the vertical-line segments 36 and 38 are aligned such that in the image frame 28, the vertical line 32 has straight edges.
Referring to FIGS. 1-2 and 4, however, a phase error between the pixel clock and the position of the beam 12 may cause the vertical line 32 to appear jagged, or in the extreme case illustrated in FIG. 4, appear as two separate vertical lines 32a and 32b. For example, assume that the pixel clock lags the beam position by a time Tlag. Therefore, during each left-to-right sweep of the beam 12, the pixel clock causes the beam to generate a respective horizontal line of a left-to-right image field 24 from time t1′ to time t2′. Likewise, during each right-to-left sweep of the beam 12, the pixel clock causes the beam to generate a respective horizontal line of a right-to-left image field 26 from time t3′ to time t4′. Consequently, as shown in FIG. 4, the image field 24 (solid line) is shifted to the right relative to the center of the scan region 31, and the image field 26 (dashed line) is shifted to the left such that these fields are not horizontally aligned as are the image fields 20 and 22 of FIG. 3. This shifting of the image frames 24 and 26 in opposite directions causes the resulting image frame 30 to be distorted in the horizontal dimension. And this distortion causes a misalignment of the vertical-line segments 36 and 38 such that the vertical line 32 now appears to a viewer (not shown) as the two vertical lines 32a and 32b, or for a smaller phase error, as a single vertical line 32 with jagged edges (not shown). Although not shown in FIGS. 3-4, distortion can occur in the vertical dimension if the pixel clock is not synchronized to the vertical position of the beam 12.